Lecture 8 Optical Sensing. ECE 5900/6900 Fundamentals of Sensor Design

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1 ECE 5900/6900: Fundamentals of Sensor Design Lecture 8 Optical Sensing 1

2 Optical Sensing Q: What are we measuring? A: Electromagnetic radiation labeled as Ultraviolet (UV), visible, or near,mid-, far-infrared (IR) 2 SI Units: Lumen (lm), Lux (lx=lm/m 2 ), Candela (cd)

3 Optical Sensors Photoresistor Photodiode Phototransistor Sparkfun SEN Fairchild QSD2030F Honeywell SDP Thermopile Infrared CMOS Image Sensors Amphenol ZTP-101T Lumex SNR On Semiconductor Samsung KAC S5K9A1

4 Proximity (Distance) Sensing Applications of Optical Sensors 4 Smoke Detector

5 Applications of Optical Sensors Controlling Gas valve with Thermopiles 5 Gas valve in Fireplace Gas valve in Waterheater

6 Smartphone/tablet Camera Applications of Optical Sensors 6 Military Night Vision Camera

7 Types of Optical Sensors 7 Quantum Detectors Photoresistor (Photocell) Photodiode Phototransistor Thermal Detectors Thermopile Image Sensors CMOS Image Sensor

8 Photoelectic Effect Photon absorption and emission of electrons 8 Einstein discovered that when high energy photons interact with electrons, it is possible for the electrons to be ejected from the material Ref:

9 Photoconductive Effect Photon absorption and increase in conductivity 9 Given small bandgap, incident photons can make the electrons jump into the conduction band. The resulting free electrons (and holes) increase conductance and hence lower resistivity of a material

10 Photoresistor (or Photocell or Photoconductor) Light Dependent Resistor (LDR) CdS (Cadmium sulfide) Track Light-controlled variable resistor The resistance of a photoresistor decreases with increasing light intensity An active semiconductor layer (e.g. CdS) that is deposited on an insulating substrate The semiconductor is normally lightly doped Less light-sensitive than photodiodes and phototransistors 10

11 LDR Circuit Example When LDR is exposed to light, LDR resistance is low which makes the base voltage of Q 1 high enough for the collector current to sink As a result LED turns on 11 When LDR is blocked, the resistance goes up (MΩ), and base voltage drops down and shuts off the transistor As a result LED turns off

12 Types of Optical Sensors 12 Quantum Detectors Photoresistor (Photocell) Photodiode Phototransistor Thermal Detectors Thermopile Image Sensors CMOS Image Sensor

13 Photodiode Photodiode p-type material is doped with boron (donor), n-type material is doped with acceptor atoms. When the light energy is greater than the band gap energy, the electrons move into the conduction band creating electron-hole pairs. In the depletion layer the electric field accelerates these electrons toward the n-layer and holes toward the p-layer. This results in a positive charge in the p-layer and a negative charge in the n-layer. If an external circuit is connected between the PN junction, electrons will flow away from n-layer, and holes will flow away from the p-layer 13

14 Photodiode Operation Photodiode Light-controlled current through a diode When exposed to light, electron-hole pairs are created Newly generated electron-hole pairs are swept away by the existing PN junction and current (also known as photo current) is created Must be reverse-biased so that the depletion region is larger Avalanche Phtodiode Solar Cell Mode 14 Photodiode Mode Responsivity (λ) =Photocurrent/Incide nt Light Power

15 Photodiode Model 15 C j R S I PD = Photo Current, I D = Dark Current=Saturation Current=Leakage Current at zero bias, C j =Junction Capacitance, R S =Shunt Resistance of zero-biased PN junction Photoconductive mode: under reverse bias, the measured output current is linearly proportional to the input optical power Photovoltaic mode: under zero bias, the current is held constant vice is restricted and a voltage builds up, dark current is kept at a minimum Ref: thorlabs.us

16 Photodiode Circuit Example Light Detector Circuits based on GaP Photodiode 16 Reverse Bias (improve bandwidth and lower junction capacitance) or Zero Bias (minimum dark current) Ref: thorlabs.us

17 Types of Optical Sensors 17 Quantum Detectors Photoresistor (Photocell) Photodiode Phototransistor Thermal Detectors Thermopile Image Sensors CMOS Image Sensor

18 Photovoltaic Effect Photon absorption and emission of electrons 18 If a high energy photon strikes an electron, the electron can escape into conduction band and creates holes and electrons (free charge carriers)

19 Phototransistor operation Large Base and Collector areas Active region operation Base is left open and is exposed to light Light causes hole electron pairs to be generated This occurs in the reverse biased base-collector junction (BCJ) The hole-electron pairs move under the electric field in the depletion region and provide the base current, causing electrons to be injected into the collector Phototransistor E 19

20 Common-Emitter (CE) Amplifier Phototransistor Example Detect light in near-infrared (λ=700 nm- 1100nm -CE: Current is amplified and Vout is generated -CC: Vout switches from Low to High state 20 Both can be operated in Active or Switch mode Common-Collector (CC) Amplifier Active mode: phototransistor generates a response proportional to the light received Switch mode: phototransistor will either be off (cut-off) or on (saturated) in response to the light.

21 Phototransistor Modes: CC Amplifier Common-Collector (CC) Amplifier (object detection, encoder) 1) Low state to a high state when light is detected 21 2) No light: Vout=0V 3) Light: base current I B is generated, which is amplified by the collector, I C is generated -with R on ~0 Ω, V out =(R E /(R E +R on ))V cc Vout=V cc 4) Mode is set by adusting R E Active Mode (V out propotional to light level): V cc > R E x I C Switch Mode: V cc < R E x I C where I C =max collector current for specific light level Ref:

22 Phototransistor Modes: CE Amplifier Common-Emitter (CE) Amplifier (compare two levels of light) 1) High state to a low state when light is detected 22 2) No light: Vout=V CC 3) Light: base current I B is generated, which is amplified by the collector, I C is generated -with R on ~0 Ω, V out =(R on /(R on +R on ))V cc Vout=0V 4) Mode is set by adusting R C Active Mode (V out propotional to light level): V cc > R C x I C Switch Mode: V cc < R C x I C where I C =max collector current for specific light level Ref:

23 Optocoupler Phototransistor Example Circuit 23 Source signal current passes through the input LED which emits an infra-red light whose intensity is proportional to the electrical signal IR light generates current in the base and it is amplified at the collector. When the current flowing through the LED is interrupted (e.g. digital data), the IR light is cut off and the photo transistor cuts off Optocouplers are used to switch transistors or other components as they provide electrical isolation between a lower voltage control signal and the higher voltage or current output signal. Examples: Microprocessor input/output switching (turn on/off motor, heater, lights), DC and AC power control

24 Types of Optical Sensors 24 Quantum Detectors Photoresistor (Photocell) Photodiode Phototransistor Thermal Detectors Thermopile Image Sensors CMOS Image Sensor

25 Thermopile Basics Thermopile Sensors Thermopiles detect thermal radiation (heat) Thermopile is a chain of serially connected thermocouples*, typically junctions. 25 a: thermopile with a reference temperature sensor attached, x and y are different materials b: micromachined thermopile sensor (note the semiconductor reference temperature sensor on the silicon frame where the cold junctions are deposited) c: sensor in a TO-5 package *Thermocouples:

26 Thermopile Operation Thermopile Thermopile Sensors Example Circuit (Melexis MLX90615) 26 Measure surface temp. of package IR light is absorbed by or emanated from the membrane and temperature of the membrane changes Since the membrane carries hot junctions, the temperature differential with respect to the cold junctions located on the frame generate thermoelectric voltage Membrane may be thermally coupled with a reference temperature sensor or attached to a thermostat having a precisely known temperature Example circuit: measured temp. is amplified and processed via DSP

27 Types of Optical Sensors 27 Quantum Detectors Photoresistor (Photocell) Photodiode Phototransistor Thermal Detectors Thermopile Image Sensors CMOS Image Sensor

28 Image Sensor Basics Charge Coupled Device (CCD) (Tutorial: CCD chip is divided into pixels* Each pixel has a potential well that collects the electrons produced by the photoelectric effect. After the exposure (incoming photons), each pixel has collected a finite amount of electrons (and hence charge) proportional to the amount of light CCD is then read out by cycling the voltages applied to the chip in a process called clocking. Clocking causes the charge in one pixel to be transferred to an adjacent pixel 28 * More at

29 Image Sensor Basics CMOS Image Sensor* Like CCDs, CMOS imagers have an array of photo diodes, one diode within each pixel Unlike CCDs, each pixel in a CMOS imager has its own individual amplifier Each pixel in a CMOS imager can be read directly on an x y coordinate system While a CCD pixel always transfers a charge, a CMOS pixel always detects a photon directly and converts it to a voltage 29 * More in

30 CMOS Image Sensor: An Example On Semiconductor KAC Resolution: 2832 (H) x 2128 (V), 6 megapixel Fabrication Technology: 4.7 μm 5T CMOS 200 MHz DDR at 400 Mbps data rate 30 * More in

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